Fix missing assert module

lib/models/email.js

@@ -4,7 +4,8 @@
 
 var Model = require('../loopback').Model
   , loopback = require('../loopback')
-  , mailer = require("nodemailer"); 
+  , mailer = require('nodemailer')
+  , assert = require('assert');
 
 /**
  * Default Email properties.
@@ -106,4 +107,4 @@ Email.prototype.mailer = mailer;
 
 Email.prototype.send = function (fn) {
   this.constructor.send(this, fn);
-}
+}